Recopying servo mechanism



March 1954 P. G. MOREL 2,673,951

I RECOPYING SERVO MECHANISM SSheets-Sheet 1 Filed June 11, 1951 March 30, 1954 P. e. MOREL RECOPYING SERVO MECHANISM Filed June 11, 1951 3 Sheets-Sheet 2 1 5 iiw W x 5 Wm w Mew I l? a T w 1 x a w a 5 W ,0, Hw INN March 30, 1954 p MOREL 2,673,951

RECOPYING SERVO MECHANISM Filed June 11, 1951 3 Sheets-Sheet 3 Audi I MM Patented Mar. 30, 1954 RECOPYING SERVO MECHANISM Pierre Louis Gabriel Morel,

Paris, France, as-

signor to Societe dElectronique et dAutomatismc, Paris, France, a corporation of France Application June 11, 1951, Serial No. 231,005 Claims priority, application France June 19, 1950 4 Claims.

The present invention is concerned with improvements to recopying servo mechanism, which amplify their performances without changing any of their qualities of faithful reproduction and servitude fidelity, independently of profile variations or design to be reproduced.

A recopying servo mechanism should be capable of providing in a homothetic way, the trajectories of a feeling member following the model, and of a reproducing member, reproducing the design or profile of the model, these two members cooperating together through a common driving system that is predetermined and practically independent of the design model; the reproducing member or the reproducing tool, is controlled in its operation by a carriage movement of at least double the range of the feeling member, and the two carriage motors have to be under the servitude of an adjustment spacing, such as t, in orientation and in modulus,

between the contact point of the feeling member on the model and the projection point of the driving device axis, of said feeling member. Such a device is usually referred to as tracing head. i r

A tracing head is normally arranged in order to deliver two measuring measures in quadrature, effectively representative of the component values of the adjustment spacing according to the tangent and to the normal at the feeling member contact point with the model profile, in the case, of course, where the adjustment spacing thus measured remains under a predetermined value, such as 60; however, the speeds of the carriage motors of the reproducer must not be under the servitude of these com ponents, but under the servitude of the vector representing the displacement speed ofthe feeling member contact point on themodel, such as 7, and it is therefore necessary to insert between thetracing head and the speed servitude devices for the carriage motors, a calculator applying a rotation matrix, such as R, onto the unitary vector of the direction of the vector of spacing a. This calculator having, on the other hand, determined the adjustment spacing, such as S=660, then applies onto vector 1' a second rotationmatrix, such as R a function of S.

In the U. S. patent application Serial No. 200,381 filed by the applicant December 12, 1950, and now abandoned. for Improvements in Reproducing Servo-Mechanism, it has beendisclosed that a particularly advantageousform, a refl rcls s a il fi ir mfoducin fide y f this matrix R (the matrix R0 being the same as matrix R) was the following:

In the example explaining the present invention, the case of such a rotation matrix willbe considered without limitation.

The present invention, in order to simplify equipments and circuits constituting such recopying servo mechanism, provides means enabling such servo-mechanism to operate with a one phase alternating current power supply, contrarily to the servo-mechanism known before, and wherein the alternating current: power supply may be dephased, at least for the calculating channel connected with the tracing head and extending to the location of the rotation control circuits of the motors, for which D. C. current control is generally used.

The present invention removes these two servitudes, thus insuring the complete homogeneity of the servitude channel from and comprising the tracing head, till the control of the carriage motors by a combination of means consisting of associating the groups of members and of circuits operating upon servitude and control signals together with reversible carriage motors.

From a co-pending application, filed concurrently with the present application by same applicant, for improvements in or relating to the control of discharge tubes, it is known, in gas or vapor discharge tubes of the type employing grid control, and receiving between the cathode and the plate a low frequency feeding alternating current voltage for the supply of the charge circuit, and connected for instance with a motor, disposed in series between the cathode and the plate with the power supply, thatto control the priming and lighting of such tubes, the control signal may be superimposed, in quadrature, as an alternating current voltage of this same low frequency, upon a carrierfloscillation of high frequency previously I modulated through a servitude signal, more particularly by a direct signal.

As this case the reversible feedingv of a charge is considered, and the control connection of the pair of tubes for this feeding direction is directly determined by adding the carrier oscillation, to the non modulated carrier oscillation in the control signal, in order that the concordance discrimination or phase opposition discrimination of thesetwo oscillationsmay be directly established for controlling the operation of one or the other control connection.

Tachometer devices are known, where the stator is composed of a magnetic core with two pairs of orthogonal poles, provided with two series connected windings in quadrature and of a ferro magnetical coil surrounding said core, and where the rotor is diamagnetic, and is disposed between said core and said coil and driven from its axis, the rotation of which is controlled, said rotor being composed of a cylindrical member with solid walls, coaxial with said core, and of a bottom plate, also solid, fast with said cylindrical memher for completing its mechanical coupling to the rotation axis and wherein an alternating current reference power supply feeds one of the windings, and having an output circuit connected to the other winding for extracting the measuring alternating current voltage.

It is known also, in such tachometer devices that theoutput circuit may be'connected to one input circuit of a voltage divider, the other input circuit of which is supplied with a reference voltage, adjustable in level and phase, extracted on said alternating current power supply of the tachometer device, preferably through a potentiometer fed by the secondary winding, the middle point of which "is grounded, of a trans- "former receiving the voltage of said alternating current power supply The spacing voltage thus defined is, after amplification, applied to one input of a phase discriminator c cuit, the other inputof which is applied with the said alternating current power supply, the consequence oi which the" error signal, in magnitude and in Q phase, is directed towards one or the other output circuit 'of the phase discriminator in order to be used for purposes of a feeding regulation "signal of one or the other rotation direction of the motorfthe speed of which is to belcon'trolled V or'put under servitude.

Such tachometer devices permit the measure- "ment of'the direction and of the magnitude of the rotation'speed of the motor to be directly obtained under the shape of an alternating .cur rent signal, the effectual value of which measures the speed, and the phase of which indicates the rotation direction; such tachometer devices, also 'enable an alternating current signal of the adjustment or error spacing to be directly obtained, by dividing the measure signal and thealternating current reference signal or posting signal of same frequency, the phase of which then indicates thedirection of the error.

The "improvements to recopying servo mechanism, according to the invention take advantage of v the above mentioned known arrangements,

in combining such arrangements with a circuit arrangement of .the measured components of an adjustment spacing S as hereinafter defined. The first important feature .in such improveiments is toprovide a tracing head with a mag- "netic circuit balanced around the mobile core carrying the feeling member, said magnetic circuit comprising two pairs of orthogonalpoles surrounding or enclosing said core, centered under balanced or rest condition. Each pair of poles is provided with windings, arranged. in oppositionand vforming two legs of a Wheatstone bridge,- the two other legs of which are made of two balancing resistors. One winding pair is fed bya one phase alternating current oscillation,-and the outputs of the measuring voltages .ofpomponents ta: and 63 are connectedwith the ..outputtemninals of the: bridge, and-circuits completed from the connection points of the windings to the connection points of the resistors.

Another important feature of said improvements, considering that the two measuring voltages are combined again, with additional setting in quadratureof the voltages one with respect to the other, in order todeliver at the terminals of a rectifier, a direct voltage determinative of the spacing a and to which is opposed to direct voltage determinative of the maximum desirable spacing 5o, hence the application of voltages :eS onto-the two outputs of said circuit, that the measuring voltage representative of 5:2 is directly considered, in being applied onto the computer, as representative of the component Uy of the displacement speed vector of the feeding member onto the model design, and that, reciprocally, the measuring voltag representative of 6y is directly considered as determinative of the component Use of the same vector; therefore, the value comparison oft e components of the displacement speed of the feeling member on the model andof the tool speed (components delivered by tachometer devices fed by the .same alternating current oscillation to the tracing :heacl such as Umx and Umy) is directly performed in addingthe components Ux and Uma: =t0xth components Up and Umy, in other extraction outputs of the voltages delivered by thetracing head,- in the suitable phases for controlling the desired rotation matrix ,of stabilization.

In order to simplify the foregoingdisclosure, it will be considered hereafter that the voltage notations willdesignate effectual values, the sign for, such an-effectual value designating a voltag in phase with the reference alternative voltage, and the sign designating a voltage phase opposed to, said reference oscillation; in practice, those phase modifications are performed in the circuits by Simple inversion of the connections.

A further feature of the improvements according to the invention, when applying a rotation matrixfunction'of S, adjustment spacing,onto the components Um and Uy of vector 7', andwith the above mentioned convention :iUm for the voltage :61! and :Uy for the voltage :auin one travel direction of the model, and vice-versa for the other travel direction, is that the" outputs of voltages is of this adjustment spacing are applied, as modulation connections, onto two modulators, supplied as for carrier frequency oscillations by suitable combinationsof thelvoltages of components U50 and U11 in phase, corresponding to those combinations.

In this respect, thepresent invention, more particularly providesfor. modulators of the well known type associated with dry type rectifiers, the above combinations of component voltages being introduced by providing multiple primaries to the input transformers of those modulators and the voltages is being applied, respectively, onto the middle points of the input and output transformers of those modulators (input secondaries and output primaries);

The voltages supplied by the comparison between the displacement speed values ofthe feeling member with the displacement speed of the reproducing tool, resulting as above mentioned from adding, insuitable phases, voltages Us: and Umzn voltages Us and Umy, are then *directly added to the output voltages of'the modulators.

The servitude'signals thus being controlledam other -fe'ature of the improvem'ents" according to the invention; and considering that the control .of the carriage motors isperformed through the .above mentioned means, is that the polarization of the discharge tubes is obtained by introducing a sole polarization voltage in both directions.

This is effected in parallel by superposition through series connections of transformer wind- .ings, the insertion of the low frequency feeding voltages in quadrature, in their suitable phases, and the automatic determination of the dischargetubes operating alternately through phase disscribed more in detail, in considering the case of recopying servo-mechanism, the stabilization matrix of which is such as above mentioned and with referenceto the accompanying drawingswhich diagrammatically show the invention and in which:

Fig. 1 illustrates the general arrangement of .the servo-:meohanism of my invention and also an elevation simplified View, of an illustrative example of a tracing head.

Figs. 2 and 3 diagrammatically show a complete diagram of an embodiment of a recopying servo-mechanism according to the invention wherein Fig. 2 indicates a right section of the tracing head, according to the invention, the diagrams being readable by placing Fig. 3 below Fig. 2 as a continuation thereof.

The tracing. head, of Fig. 2, comprises a magnetic core 13, carried by a rigid axis 75 carrying at its end a feeling member 2 supposed to follow the profile of a model I. This magnetic core is carried by a calling back arrangement such that in absence of unbalanced condition of the mobile equipment of the tracing head, an electric balance is obtained, the interferric space of the two pairs of poles 69-70 and 'H- l2 carried by the annular frame 16, being equal, under rest condition.

Poles 69 and 10 carry two opposed windings forming two legs of a Wheatstone bridge, the two other legs of which are constituted by the resistors of a potentiometer 81; a bridge diagonal,

at the potentiometer terminals, is fed by the secondary of a transformer 86, the primary of which receives an R. F. oscillation from a generator B, such as an alternator or convertor, it-

self fed at I by the mains. By way of example,

it may be cnsidered that this converter delivers a one phase alternating current voltage at 1000 cycles mains. For simplicity sake, said R. F.

voltage will be hereafterreferred to as a carrier oscillation.

Poles H and 12 are equipped in the same way with opposed windings constituting two legs of a Wheatstone bridge the two other legs of which are constituted by the resistors of a potentiometer 88. The two circuits are arranged in parallel as indicated by connections 26 and I21.

The lines of :force of the R. F. magnetic field close in four magnetic circuits closed through core 13, the magnetic bridges are balanced and therefore any parasitic variation of permeability, temperature, etc., is compensated.

,When the magnetic core is underrest condition, the voltages developed at the diagonals of they Wheatstonecbridges; ending at the transformer primaries 9 and 8, respectively are equal to zero.

When said core, driven by the displacement of the feeling member 2 along the profile of model i reaches a thrown off center position, two unbalanced voltages appear, with the same carrier frequency, at the diagonals, the eifectual' values of which are proportional to the spacings, according to axis X-X and YY, not represented, of the pairs of poles H-'l2 and Bilill, and these voltages are of same phase or of opposed phase to that of the carrier oscillation according to the profile, so that the voltage developed across the terminals of transformer 8 is normally inclusive of the component i621, and the voltage developed across the terminals of transformer 9 is inclusive of the component :61: of the above mentioned vector 6.

In such a balanced tracing head, the magnetic core 13 must always be called back towards its center position; several mechanical arrangements may be taken into consideration for that purpose, and at Fig. 1 is diagrammatically shown by way of example such a call back arrangement.

Axis 14 is extended rearwards with respect to core 13 and is provided with a plate 18, which by means of bearing balls (three bearing balls at 120 for example) exert a pressure in resting against a circular ring which rests itself, through three bearing balls, also spaced by 120 but shifted in relation with the first ones, against one internal flange ll of the casing of the tracing head; the directions 'of the bearing balls casing are orthogonals. The rear end of axis 14 slides into a support 8|, as indicated at 80, and this support is constantly under the pressure of a spring 82 bearing on plate 18 so that its point 83 bears against the conical wall 35 of a hard metal seat inserted into the corresponding face of the housing. Thus, axis 14 may move without rotation according to any control transmitted from the feeling member, while being constantly urged towards its center position.

Referring now more particularly to the arrangement shown in Fig. l, the model I is followed by the feeling member 2. The reproduction is shown at 3 and the reproducing tool at 4. The mechanical link 5 insures the simultaneous driving of parts 2 and 4, independently of the profile, for instance at a uniform speed, through means not shown and out of the scope of the present invention.

The homothety of displacements of feeling member 2 on the model and of the tool 4 on the reproduction, is obtained through the servomechanism control of the two carriage motors 45 and 46, linked to said tool through themechanical link.

Voltages its and icy delivered by the tracing-head are respectively amplified at H and H1 through A. C. amplifiers provided with counterreaction circuits #2 and it. These amplified voltage have as provided for in the present invention, their references inverted, that is to say that for a voltage may, it considered that the amplified voltage is :Uac and that for a voltage i-tzc it is considered that the amplified voltage is'i-Uy; this simple consideration, enabling the passageof vector 6 to vector 1-, for a predetermined travel direction of the feeling member, and vice versa for the other travel direction, and 'iUa: corresponding then to :6 and :Uy corresponding to $53!. The adjustment spacing is is controlled as is applied onto a divider cults i! and 20, respectively,

and: 2|, respectively,

aeraesi hereunder described; from theoutput circuit [4 of amplifier I0, through lead 23, the voltage Um point to which is returned a voltage of same value as Uy, derived, for this purpose, from circuit ii in the output of amplifier II, but set in quadraturain lead 19,

through inserting a-de'phasing network 24. (Such setting intoquadrature may just as well be performed on voltage Um.)

The divider point 25 connects to a rectifier 26, the output of which opposes at 28 a direct voltage of reference spacing 60, derived, for example from a rectifier 2'! fed by themains 1. It is to be noticed here, that the various necessary voltages are all formed as from the power supply network in order thatany variation of this network will be-noticed' in the derived voltages, and that in this way the necessity of correcting the network variation at the comparison locations of the servo mechanism is eliminated.

'From point 28, the voltage 6-60 is amplified at'29' and from the output 33 of i this amplifier are connected the two modulation circuits 3| and 32 applying voltage is onto the two modulators 33 and 34.

These modulators each receive, as carrier niodulation itself, an oscillation derived through combination of voltages Us and Uy. In the example considered, the oscillation applied onto modulator 33 results fromv adding, throughcirvoltages +Ug and Um, whence the oscillation applied onto modulator 34 results from adding, through circuits l8 voltages +Ux and +Uy. The output of modulator 33 is therefore a voltage at the frequency of the carrier oscillation, with an effectual value S(UxUy) whence the output of modulator 34 is a voltage of the same carrier frequency, with an effectual voltage S(U:v+Uy).

The modulated output voltage of 33, is supplied through link 35, connected with the input ofanamplifier 31 with a counterreaction circuit 33 and the modulated output voltage of 34, is supp-lied through link put of an amplifier 38 with a counterreaction circuit 40. The other voltages necessary for controlling servitude signals are impressedupon the respective inputs of these amplifiers, that is,

voltage Uyonto amplifier 31 and voltage Ur onto amplifier 38, through respective leads i5 and 22, on the other hand, the respective measure voltages Umy and -Um:c of the rotation speed of the motors and 46- are also brought into said inputs through leads 4| and 42.

These measure voltages are controlled through tachometers 43 and 44, respectively driven by mechanical links 41 and 48 driven from the rotation axis of the motors 45 and 46; these tachometersare fed by a reference voltage, adjusted at 50 and applied onto them through lead 49. This reference voltage is of the carrier frequency, being impressed through lead 51 from generator 6.

The servitude signals being thus controlled are respectively applied onto phase discriminators 53 and 54, with each one of two distinct outputs 5l-58 and Eli-; these signals are respectively directed towardsone or the other of these outputs by comparison of their phase with that of the reference oscillation, carrier, applied onto these discriminators through leads 55 and 55.

Each pair of outputs 5'l-58 and 59+-60 are connected to a control circuit of network power supply, and to inductor windings -66- and 35, connected with the in- A 61-458 respectively of reversible rotation. motors 45 and 46; these circuits are indicated at El and 62 and are fed: by said network voltage through leads 63 and 64.

Detailed illustration of the members and circuits of such a servitude channel for. recopying are shown in Figs. 2 and 3. As far aspossible; the reference numerals of Fig. 1 are shown in Figs. 2 and 3 applied to like parts,

The transformers B and 9 for the voltages delivered by the tracinghead'are arrangedto apply these voltages onto tube amplifier stages I0: and Ii, the high frequency of which is: supplied through the primaries of their output transformers it and 15 with multiple'secondaries, andlthe stabilization of which through high counter-reaction is precisely insured by thesecondaries l2 and i3 having one end grounded, and. the other connected to the secondary of their respective input transformer 8 01 9.

The voltages appearing at the terminal of secondary winding i9- of transformer I5, and secondary winding 23 of transformer M, are im'- pressed in quadrature on the phase shifting network comprising resistor-capacitors 24 and the resultant voltage is rectified through bridge 25.

From the output terminal of bridge 29 two leads are connected to symmetrical direct current am:- plifier 29. In the negative leadfrom the output of the bridge there is provided a counter-battery voltage 23 supplying opposition voltage to establish the maximum adjustment desired for admisspacing; this counter-battery may of course be constituted of a rectifier fed by the mains, as over mentioned. A1; the output 33' of amplifier 23, thus appears, on leads is and 90, the adjustment spacing voltages :S. Leads 89 and 30 are respectively connected to the middle pointstl of the input and output winding of modulator 33 and onto the middle points 32 of the corresponding winding of modulator 34, These two modulators are of the conventional-type. The rectifiers are preferably of the dry type.

The input transformer of modulator 33 isprovided with two primary windings ll and 23, respectively fed by secondary winding of transformers l5 and 14, in relative phase opposition; the input transformer of modulator 34 is provided with two primary windings i8 and 2 i, respectively fed by the samesecondary windings of transformers l5 and i4, but in phase concordance. The modulated voltages thus obtained are transmitted through the secondary windings 9| and 92 of the modulators output transformers 35 and 33.

The output of generator 6 of carrier frequency is shunted at 5! and applied in parallel through this shunt onto windings l2l and I22 of tachometer devices 43 and M-Jig. 3-as reference oscillations, calibrated by a posting potentiometer after phase shifting of 90 in E25.

potentiometer may furthermore consist, when necessary, in a potentiometer fed'as from a transformer located at the end of shunt 5 I, provided with a middle point of secondary winding to the ground.

The two windings I25 and IE4 of the two tachometers, respectively orthogonal with the windings fed by the reference oscillation, therefore, deliver measure voltages of respective rotation speed of motors 45 and 48, on circuits ll and 42; these voltages are of the carrier frequency and are invphase or in phase opposition with this oscillation.

Through circuit- 4 l, the-winding l-23'oftachomtete'r 43 of motor 45 (carrying the tool along axis of the Y) is connected in subtractive series (phase opposition) to the secondary winding I6 of transformer I5, in an outlet lead of the secondary winding of modulator 33. Thus, between terminals 93, is established a servitude voltage of the shape S(Ux-Uy) Uy-Umy and applied onto the input transformer 95 of the amplifying stage 31. This amplifying stage is arranged in an analogous way to that of the output amplifiers of the tracing head, that is to say, that it comprises a tube stage 9? fed with high voltage through the primary winding of its output transformer, whence the desired high counterreaction is supplied by a secondary winding 39 of this output transformer, one end of which is grounded and the other connected to the secondary winding of the input transformer 95 arranged in series with the tube stage 07.

Similarly, the winding I24 of tachometer 44 of motor 56 (carrying the tool along the axis of the X) is connected in subtractive series, through circuit 42, to the secondary winding 22 of transformer I 4, in an output circuit of the secondary winding 92 of modulator 3 3. Thus is established across terminals 94, a servitude voltage of the shape S(U:v+Uy) +Ucr-Umx applied onto the input transformer 95 of the amplifier stage 38. This amplifier stage is similar to stage 31 and comprises a tube stage 98 and a counterreaction secondary winding 50.

The amplifier stages 37 and 53 connect with the phase discriminator circuits 53 and 55, in order to direct the servitude high frequency signals, respectively on one or the other pair of dis charge tubes I -l 05 and I 01-!08 fed according to either rotation direction of motor 35, and on one or the other pair of discharge tubes I09-I I0 and I I II I2 fed according to either rotation direction of motor 05.

From these discriminators, the fixed polarization voltage of the discharge tube control grids is applied, through battery IOI, onto the middle points 99 and I00 of the secondary windings of the output transformers of stages 31 and 38, constituting the discriminators input.

The discrimination takes place through the superposition, in each starting lead of these secondary windings, of the reference carrier voltage, non-modulated, introduced by the series arranged secondary windings 55 and 56 (two secondary windings for each servitude way) of transformer 22 for deriving the carrier oscillation from shunt 5|. In this way, the phase discrimination is automatically performed in the four circuits 51, 58, 59 and 60 which lead to the control grids of the discharge tubes in each pair.

It is necessary, in order to control the discharge tubes, to superpose on to the high frequency servitude voltages, other voltages of the network power supply frequency, in quadrature with said voltage as applied to the plate transformers II'I to I20 of the pairs of tubes, this voltage in quadrature progressing for one tube of each pair and being delayed for the other tube.

This is accomplished in a simple way, by deriving the power supply voltage 1, and in phase shifting it (Fig. 3) by setting the voltages into quadrature in network I03; after which the phase shifted voltage is applied onto the primary winding I 04 of a, transformer provided with eight secondary windings 636G, alternately series connected, in concordance and phase opposition, in the connecting leads starting from the four points 51-60 and reaching the control grids of tubes I05 to H2, respectively; quadrature is applied through the direct lead of the superior secondary winding, in additive series, onto the control grid of tube I05, and through the direct lead, in subtractive series, of the immediately inferior winding onto the control grid of tube I05 for the signal appearing at 51 and so on.

i As usual, the feeding of windings 6556 of motor 45 takes place as from the middle points H3 and lid of transformers III to H8, and the feeding of the windings of motor 40, as from the middle points H5i I5 of transformers H9 and I 20.

While Ihave described my invention in certain preferred embodiments I realize that modification may be made and I desire that it be understood that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

What I claim is:

1. A reproducing servo-mechanism system in which the similarity of the trajectories of a feeling member on a, model and of a reproducing member on a blank is assured by controlling the speeds and direction of rotation of two motors moving the reproducing member along two directions at right angles, a feeling member, a tracer head connected to said feeling member, a reproducing member, a pair of motors, a tachometer connected to each of said motors, said tracer head and said tachometers being supplied with a voltage the frequency of which is higher than that which is supplied to said motors, said head delivering two signals in connection with the displacement of the feeling member and said tachometers delivering two signals in connection with the speed and rotation of said two motors, a pair of modulators, each of said modulators receiving a carrier oscillation at the said higher frequency and adapted to modulate said carrier oscillation by the difference signal between a fixed D. C. voltage representing the maximum regulating difference, and a voltage representing the real difference obtained by mixing one of the two signals from the tracer head set in quadrature phase relation with the other signal, means for rectifying the mixed signal, a pair of mixing elements, each receiving the output signal from one of said modulators, one of said tracer head and the moves the reproducing member in a direction at right angles to the component of the displacement of the feeling member represented by the signal from the tracer head, phase discriminating elements for the output signals from the mixing elements, said phase disemploying a reference phase oscillation at the said high frequency and twin channels from each of the output channels of the phase discriminators, a pair of discharge tubes each including an ignition electrode, said discharge tubes controlling one of the directions of rotation of one of the motors, said twin channels from each of the output channels of the phase discriminators being respectively connected with said ignition electrodes and receiving superimposed thereon a signal at the motor supply frequency in quadrature phase relation with the supply frequency but in phase opposition with the signal applied to the other twin channel from the same output channel.

2. A reproducing servo-mechanism system as set forth in claim 1 in this voltage in j which the carrier Osci1-- amass 11 lation applied to the modulators is constitutedby the signals representing the displacement of the feeling member, in phase concordance for one modulator, and in phase opposition for the other.

3. A reproducing servo-mechanism system as set forth in claim 1 in which the tachometers each comprise a stator with two pairs of poles at right angles, each carrying a winding, to one of which the high frequency voltage is supplied, and from the other one of whichthe voltage measuringthe velocity of the motor is derived.

4. A reproducing servo-mechanism system as set forth in claim 3 in which the tachometers each comprise a stator core surrounded by a stationary ring, both formed from ferromagnetic material and a rotor of diamagnetic material mountedin the air gap between said core and said ring, said rotor being ,mechanically con nected to rotate with the associated motor.

PIERRE LOUIS GABRIEL MOREL.

References Cited in the file of this patent UNITED STATES PATENTS 

